1. Field of the Invention
The present invention relates to an exhaust manifold for an internal combustion engine or, more particularly, to an exhaust manifold comprising a plurality of exhaust pipes extending from exhaust ports of an internal combustion engine and a merging portion of the exhaust pipes with an oxygen sensor arranged therein, wherein the exposure of the oxygen sensor to the exhaust gas is improved for improving the durability of the oxygen sensor
2. Description of the Related Art
Conventional exhaust manifolds for internal combustion engines are made of cast iron having a comparatively large weight. In order to reduce the weight and the thermal capacity of the engine, however, the current practical trend is toward the use of an exhaust manifold comprising a plurality exhaust pipes of stainless steel mounted to exhaust ports and combined at a merging portion. An electronically-controlled internal combustion engine, on the other hand, is required to detect the air-fuel ratio in the exhaust gas to control the fuel injection amount. With the electronically-controlled internal combustion engine, therefore, an air-fuel ratio sensor (usually, an oxygen sensor) is mounted at the merging portion.
For this electronically-controlled internal combustion engine, the oxygen sensor is desirably exposed uniformly to the exhaust gas from each cylinder. In view of this, various shapes of the merging portion of the exhaust manifold have been proposed to assure that the oxygen sensor is uniformly exposed to the exhaust gas from each cylinder. According to Unexamined Patent Publication (Kokai) No. 7-97921, for example, each exhaust pipe protrudes into the merging portion with the forward end thereof curved toward the oxygen sensor or an inclined wall is formed at the exhaust gas outlet of each exhaust pipe.
The shape of the exhaust manifold disclosed in Unexamined Patent Publication (Kokai) No. 7-97921, however, is liable to reduce the durability of the oxygen sensor. The reason for the low durability is that the oxygen sensor is mounted at the merging portion in parallel to a plurality of exhaust pipes so that, in the case where the end of each exhaust pipe is curved toward the oxygen sensor or the inclined wall is formed at the exhaust gas outlet of each exhaust pipe, the high-temperature high-velocity exhaust gas directly strikes the oxygen sensor located in the neighborhood of the exhaust pipe outlets.